Inner roller and outer roller in internal-meshing planetary gear construction, and methods of fabricating them

ABSTRACT

An inner roller 30 (in FIG. 2) for use in an internal-meshing planetary gear construction has its inner-peripheral wall 32 formed into a shape which is brought closer to a regular polygon of n sides (where n denotes an integer of at least 3, and n=3 holds in the illustrated example). Lubricating oil is reserved in clearances H1 defined near the vertices of the regular triangle, and an inner pin 7 is retained in light touch with the three middle points P1-P3 of the respective sides of the regular triangle. Thus, the reservation of the lubricating oil and the retention of the inner pin 7 free from backlash are made consistent, thereby to relieve the backlash of the planetary gear construction and to enhance the durability thereof. The shape of the inner-peripheral wall 32 closer to the regular polygon of n 7 is obtained in such a way that the inner-peripheral wall 32 is subjected to a burnishing work in the state in which depressive forces (deforming forces) based on chucking jigs are applied to the circumferential equidistant positions of the cylindrical blank of the inner roller 30, and that strains (residual strains) remaining after the burnishing work are further enlarged by a heat treatment.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.08/872,386, filed Jun. 10, 1997. The subject matter of Application Ser.No. 08/872,386 is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an inner roller or an outer roller inan internal-meshing planetary gear construction, and a method offabricating the inner or outer roller.

2. Description of the Prior Art

There has heretofore been extensively known an internal-meshingplanetary gear construction having a first shaft, eccentric elementswhich are rotated by the rotation of the first shaft, external gearswhich are assembled so as to be respectively rotatable eccentrically tothe first shaft through the corresponding eccentric elements, aninternal gear with which the external gears are "in internal mesh" (thatis, with which the external gears mesh internally of this internalgear), and a second shaft which is connected to the external gearsthrough device for transmitting only the rotational components of theexternal gears on the axes thereof.

An example of the construction in the prior art is illustrated in FIGS.6 and 7. The prior-art example is such that the construction is appliedto "reduction gears" by employing the first shaft as an input shaft andthe second shaft as an output shaft and by fixing the internal gear.

Eccentric elements of eccentricity e 3a, 3b are snugly fitted on theinput shaft 1 with a predetermined phase difference (180° in thisexample) set therebetween. The eccentric elements 3a and 3b are integralin this example. Two external gears 5a, 5b are mounted on the respectiveeccentric elements 3a, 3b through corresponding bearings 4a, 4b. Each ofthe external gears 5a and 5b is formed with a plurality of inner rollerholes 6 (6a and 6b), in each of which an inner pin 7 and an inner roller8 are inserted.

The inner pin 7 is enclosed or concealed with the inner roller 8 for thepurpose of dispersing slips during the operation of the planetary gearconstruction, that is, the slips between the inner pins 7 and theexternal gears 5a, 5b are dispersed into the slips between the innerpins 7 and the inner rollers 8, and the slips between the inner rollers8 and the external gears 5a, 5b.

The inner pins 7 and inner rollers 8 which penetrate through theexternal gears 5a, 5b are secured or fitted in the flange portion of theoutput shaft 2.

The 2 (two) external gears 5a, 5b (in a double-row structure) arechiefly intended to enlarge a transmission capacity, to maintain astrength and to hold a dynamic rotational balance.

External teeth 9 of trochoidal tooth profile, circular-arc toothprofile, or the like are provided at the outer periphery of each of theexternal gears 5a, 5b. The external teeth 9 are in internal mesh withthe internal gear 10 fixed to a casing 12.

The internal teeth of the internal gear 10 are concretely constructed ofouter pins 11. The outer pins 11 are loosely fitted in outer pin holes13, and are held easy of rotation. Each of the outer pins 11 issometimes enclosed with an outer roller 14 as shown in FIG. 8 by way ofexample. Thus, slips during the operation can be dispersed (that is, theslips between the outer pins 11 and the outer pin holes 13 in FIG. 7 canbe dispersed into the slips between the outer pins 11A and the outerrollers 14, and the slips between the outer rollers 14 and the outer pinholes 13 as understood from FIG. 8).

The operation of the exemplified reduction gears will be brieflyexplained. When the input shaft 1 is rotated one revolution, theeccentric elements 3a, 3b is also rotated one revolution. When theeccentric elements 3a, 3b perform one revolution, the respectiveexternal gears 5a, 5b are about to rock (or swing) and rotate around theinput shaft 1. Since, however, the rotations of the external gears 5a,5b on the axes thereof are restrained by the internal gear 10, theexternal gears 5a, 5b perform almost only the rocking in internal meshwith the internal gear 10.

Now, supposing by way of example a case where the number of teeth ofeach of the external gears 5a and 5b is N and where the number of teethof the internal gear 10 is (N+1), the difference between the numbers ofteeth is 1 (one). Consequently, each time the input shaft 1 is rotatedone revolution, the external gears 5a and 5b shift (or rotate on theirown axes) to the amount of one tooth relative to the internal gear 10fixed to the casing 12. This signifies that speed of one rotation ofinput shaft 1 is reduced to speed of -1/N rotation of the external gears5a, 5b. Here, the minus sign indicates the reverse rotation (or therevolution in the reverse direction).

The rotations of the external gears 5a, 5b have the rocking componentsthereof absorbed by clearances defined between the inner roller holes 6and the inner rollers 8, and only the rotational components thereof ontheir own axes are transmitted to the output shaft 2 through the innerpins 7 inserted in the inner rollers 8. As a result, speed reduction ata reduction ratio of -N is eventually accomplished.

By way of example, accordingly, a geared motor of great reduction ratiocan be obtained with only one stage of reduction mechanism by combiningthe reduction gears of the internal-meshing planetary gear constructionwith an electric motor.

In the prior-art example, the internal gear of the internal-meshingplanetary gear construction is fixed, and the first shaft and secondshaft are respectively employed as the input shaft and output shaft.However, reduction gears can also be constructed by fixing the secondshaft, employing the first shaft as an input shaft and employing theinternal gear as an output shaft. Further, speedup gears can also beconstructed by inverting the input/output relations of each of suchreduction gears.

Besides, in the prior-art example, the eccentric elements are directlymounted on the outer periphery of the first shaft. In this regard, therehas also been known a construction of the type wherein the first shaftis dispersed or divided into "three first shafts" through spur gears,eccentric elements are respectively mounted on the dispersed firstshafts, and the external gears are rockingly rotated through theeccentric elements. The present invention is applicable even to theinternal-meshing planetary gear construction of such a type without anyproblem.

Meanwhile, as exaggeratedly shown in FIG. 9, a clearance δ1 is definedbetween the outer periphery of the inner pin 7 and the inner peripheryof the inner roller 8. Besides, as exaggeratedly shown in FIG. 10, aclearance δ2 is defined between the outer periphery of the outer pin 11Aand the inner periphery of the outer roller 14. Each of the clearancesδ1 and δ2 serves to ensure the formation of a lubricating oil filmbetween the two members, and to allow the smooth slip between themembers touching with each other.

However, when such a clearance δ1 or δ2 is defined, the problem arisesthat backlash develops between the inner pin 7 and the inner roller 8 orbetween the outer pin 11A and the outer roller 14, with the result thatthe whole gear transmission mechanism undergoes backlash. This incursthe disadvantage that, when the rotation on one side is to betransferred to the rotation on the other side, the rotation on thedriving side does not immediately appear as the rotation on the drivenside. Hereinbelow, such a delay in response shall be termed "angularbacklash".

In a case where the internal-meshing planetary gear construction is usedas the control mechanism of, for example, a servomotor, the angularbacklash degrades the control precision thereof. Various causes areconsidered for the occurrence of the angular backlash in theinternal-meshing planetary gear construction. As contrivances foreliminating such angular backlash, there have hitherto been knownvarious structures, for example, one wherein the external gears, theinternal gear, etc. are bisected for forward rotation and for reverserotation, and one wherein the roles of the external gears, the internalgear, etc. are allotted to the forward rotation and to the reverserotation (refer to Japanese Patent Applications Laid-open No.106744/1984, No. 113340/1984, No. 115743/1984, No. 208366/1984, and soforth).

In addition, the inventors have ever proposed an expedient in JapanesePatent Application No. 86571/1985 (corresponding to Japanese PatentApplication Publication No. 86506/1993) as a method of minimizing aclearance relevant to an outer pin and an outer pin hole (in aninternal-meshing planetary gear construction of the type which has noouter roller).

In any of the known examples, however, note has never been taken of theclearance δ1 between the inner pin and the inner roller or the clearanceδ2 between the outer pin and the outer roller for the purpose ofrelieving the angular backlash. It has been the actual circumstancesthat quite no measure is taken to counter the angular backlashdeveloping in the clearance δ1 or δ2.

The reason therefor is that the clearance δ1 or δ2 has been consideredunremovable (as an indispensable constituent) because of the followingrequisites: 1 Predetermined lubricating oil needs to be always reservedbetween the inner pin and the inner roller or between the outer pin andthe outer roller. 2 Even at the appearance of the state in which theaxes of the inner pin and the inner roller or those of the outer pin andthe outer roller have deviated due to a machining error, an assemblageerror, or the deformation of the two members during the transmission ofpower, both the members need to be slipped smoothly.

Incidentally, the use of a material of low friction and good affinity,for example, white metal or fluorocarbon resin is also considered as amethod which ensures the favorable slides of the two slide memberswithout defining the clearance. In general, however, a great torque withthe torque of the input shaft amplified several times to 100 or moretimes, acts on the inner roller or outer roller of the internal-meshingplanetary gear construction. Therefore, a material of high hardness andhigh strength must be used from the viewpoint of durability, and theabove method cannot be adopted in many cases.

Further, in this regard, the material of high hardness and high strengthneeds to be wrought at a high precision, so that the inside and outsidediameters of the inner roller or the outer roller must be finished by"grinding". Especially in case of finishing the inside diameter by thegrinding, a finish roughness is limited (to 2-3 [μm] as an economicalvalue) for the reason that the grinding is, to the last, a work forshearing the crystal grains of the material. This has led to therequisite that the existence of the clearance δ1 or δ2 having a certainmagnitude is indispensable for keeping an oil film under the conditionof the roughness.

For the reasons thus far explained, the clearance δ1 between the innerpin and the inner roller or the clearance δ2 between the outer pin andthe outer roller has hitherto been thought indispensable. Accordingly,the occurrence of the angular backlash ascribable to the clearance hasbeen thought inevitable.

SUMMARY OF THE INVENTION

The present invention has been made in view of the problems of the priorart as stated above, and has for its object to provide an inner rolleror an outer roller in an internal-meshing planetary gear construction inwhich angular backlash can be rendered much less than in the prior artwithout posing any new drawback, by reconsidering on the basis of adifferent conception the shape of the cross section of the innerperiphery of the inner roller or outer roller, as has hitherto beenthought to (naturally) require a work into a shape closest to a truecircle to the utmost (namely, a work at the highest possible degree ofcircularity).

Another object of the present invention is to provide a method which isthe most suitable for actually fabricating the inner roller or outerroller.

The objects are achieved by structures defined in Claims 1 and 2, asregards the inner roller in the internal-meshing planetary gearconstruction. Quite similar structures defined in Claims 3 and 4 areapplied to the outer roller.

The cross section of the inner-peripheral wall of the inner roller orouter roller according to the present invention has a shape which isbrought closer to a "regular polygon of n sides" unlike a true circle(more exactly, the closest possible true circle) in the prior art.

Here, letter n denotes an integer of at least 3. In the concrete,accordingly, the concept of the "regular polygon of n sides" shall covera regular triangle, a regular tetragon, a regular pentagon, a regularhexagon, . . . .

Furthermore, as described later, in the present invention, from the sameviewpoint, a rhombus (practically ellipse) may well be adopted.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become more apparent from the following description ofthe invention taken in conjunction with the accompanying drawings,wherein like reference characters designate the same or similar parts,and wherein:

FIG. 1 is a front view of an inner roller which is applied to aninternal-meshing planetary gear construction according to the presentinvention;

FIG. 2 is an enlarged sectional view taken along line II--II in FIG. 1;

FIG. 3 is a flow sheet showing an example of a method of fabricating aninner roller the inner-peripheral wall of which is brought closer to aregular polygon of n sides;

FIGS. 4A-4E are flow diagrams for explaining the aspect of performanceof a method of fabricating an inner roller according to the presentinvention;

FIG. 5 is an enlarged sectional view corresponding to FIG. 2, forshowing another example of inner-roller.

FIG. 6 is a front view, partially broken away, for explaining thestructure of an internal-meshing planetary gear construction in theprior art;

FIG. 7 is a sectional view taken along line VII--VII in FIG. 6;

FIG. 8 is a partial enlarged sectional view for explaining the structureof an internal-meshing planetary gear construction which has an outerroller;

FIG. 9 is an enlarged sectional view corresponding to FIG. 2, forexplaining the relationship between an inner roller and an inner pin inthe prior art; and

FIG. 10 is an enlarged sectional view corresponding to FIG. 2, forexplaining the relationship between the outer roller and an outer pin inthe prior art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, the aspects of performance of the present invention will bedescribed in detail with reference to the drawings.

The present invention is pertinent only to an inner roller or outerroller for use in an internal-meshing planetary gear construction, andthe structure of the whole internal-meshing planetary gear constructiontoward which the present invention is directed is not especiallydifferent from the prior-art structure. Accordingly, the structure ofthe whole internal-meshing planetary gear construction shall be omittedfrom description because it has already been detailed.

FIG. 1 is a front view of an inner roller 30 according to the presentinvention, while FIG. 2 is a sectional view taken along line II--II inFIG. 1.

The inner roller 30 is formed into the shape in which the cross sectionof the inner-peripheral wall 32 of the cylinder of this inner roller isbrought closer to a regular triangle from the true circle.

In FIG. 2, a circle depicted by a two-dot chain line indicates the outerperiphery of an inner pin 7 corresponding to the inner roller 30. Asseen from the figure, the outer periphery 7A of the inner pin 7 isformed into the true circle as in the prior art. This outer periphery 7Aof the inner pin 7 lie in light touch with the middle parts P1-P3 of therespective sides of the regular triangle. The clearance H between eachof the vertex parts of the regular triangle and the outer periphery 7Aof the inner pin 7 is concretely set at 5-20 [μm], preferably 10 [μm] orso, in a case where the outside diameter of the inner pin 7 is on theorder of 17 [mm] by way of example.

Owing to the inner-peripheral wall 32 of the inner pin 30 endowed withsuch a shape, spaces H1 shown in FIG. 2 function as vacancies in whichlubricating oil is reserved. Besides, since the inner pin 7 lies intouch with the inner roller 30 at the middle parts P1-P3 of theinner-peripheral wall 32 of this inner roller 30, the axis O1 of theinner roller 30 and that O2 of the inner pin 7 do not deviate unless anespecially great force acts on the member 30 or 7. Accordingly,"backlash" does not develop as a rule. Therefore, the "angular backlash"ascribable to the clearance δ1 as in the prior art does not develop,either.

Further, even if a machining error, a) an assemblage error or the likehas been involved, b) the member 30 or 7 has been elastically deformedduring the transmission of power, or c) the respective axes O1 and O2 ofthe inner roller 30 and inner pin 7 have deviated due to an unexpectedgreat external load, such conditions can be flexibly coped with by someshifts of the touch positions of the inner roller 30 and inner pin 7because these members 30 and 7 lie in touch slightly at only the threepoints of the middle parts P1-P3. That is, even when such circumstanceshave occurred, the inner roller 30 and inner pin 7 can keep a smoothslip.

Owing to the adoption of the inner roller (or outer roller) of thestructure described above, accordingly, the angular backlash can berelieved in such a way that the inner roller (outer roller) and innerpin (outer pin) are reliably kept coaxial with the lubricating oilreserved in a large quantity, that is, without degrading the durabilityof both the associated members. Furthermore, the favorable slipcharacteristics can be ensured even in the presence of the machiningerror, the elastic deformation during the operation of the planetarygear construction, the elastic deformation ascribable to the externalload, or the like, so that increase in a power loss is not incurred.

Next, a method of fabricating the inner roller or outer roller in such ashape will be concretely described.

In forming the inner-peripheral wall into the shape which is closer tothe regular polygon of n sides, it is impossible to adopt the(true-circle work) method of cutting or grinding as in the prior art.Besides, a method illustrated in FIG. 3 by way of example isconceptually considered. In this method, a circle R1 to be inscribed tothe regular polygon of n sides is first formed by cutting, and thespaces H1 extending along the axis of the inner roller 30 aresubsequently formed by cutting. It is next to impossible, however, toactually form the shape closer to the regular polygon of n sides withaccuracy by the use of this method.

The inventors have therefore originated a method in which a burnishingwork attended with chucking strains and a heat treatment attended withthermal strains are combined so as to positively utilize the workingstrains and the thermal strains.

FIGS. 4A-4E illustrate steps for fabricating the inner roller (or outerroller). Incidentally, the contraction scales of these figures are notthe same because preference is taken of clear illustration.

As shown in FIG. 4A, an inner-roller blank 30a being elongate is severedat predetermined lengths L into inner-roller blanks 30b.

Subsequently, as shown by an enlarged cross section in FIG. 4B, thecentral part of the inner-roller blank 30b is cut away by any suitablemethod, for example, boring, reaming or turning. Then, an inner-rollerblank 30c in the shape of a cylinder is roughly formed.

The above steps explained are similar to the steps in the prior art. Inthe prior art, a finishing process for the inner-peripheral wall 32c ofthe inner-roller blank 30c is thereafter initiated to lathe with aturning. In contrast, according to the present invention, the finishingprocess is not performed, but the inner-peripheral wall 32c issubsequently made a mirror-finished surface by a burnishing work.

The "burnishing work" is a working method in which a metal surfacehaving ruggedness is press-rolled (or crushed) by a burnishing roller soas to be finished into a smooth surface like a mirror, and which isknown in itself. The burnishing work is a kind of plastic deformationwork, and it differs from a work such as cutting in which the crystalgrains of metal are shaved off by shearing. It is therefore possible toobtain the inner-peripheral wall which is very smooth in point ofsurface roughness.

In executing the burnishing work, the inner-roller blank 30c is fixed byany expedient, and the burnishing roller not shown is inserted into thethrough hole 31c of the blank 30c.

In the present invention, the fixation of the inner-roller blank 30c ispositively utilized for forming the regular polygon of n sides.

More concretely, the burnishing work proceeds as explained below. First,as shown in FIG. 4C, the outer periphery 33c of the inner-roller blank30c is chucked at the circumferential equidistant positions of thisblank 30c (at the positions of this blank 30c at regular angularintervals of 120° in an exemplary case where the cross section of theinner-peripheral wall of an inner roller is brought closer to the shapeof a regular triangle) by chucking jigs 50 so as to generate depressiveforces F of predetermined magnitude directed radially inward of thisblank 30c. Thereafter, the inner-peripheral wall 32c is subjected to theburnishing work (true-circle work) in the state in which the depressivechucking is done (that is, in the state in which the inner-roller blank30c is deformed radially inward at the circumferential equidistantpositions by the chucking).

The depressive chucking is released after the burnishing work. Then, asshown in FIG. 4D, an inner-roller blank 30d formed with aninner-peripheral wall 32d deformed inversely to the depressivedeformations is obtained owing to working strains (residual strains). Atthis stage, the inner-peripheral wall 32d has the shape which is closerto the regular triangle, and the surface thereof is a mirror-finishedsurface which is very smooth.

In this aspect of performance, the inner-roller blank 30d issubsequently heat-treated so as to enlarge the strains still more,thereby to obtain an inner-roller blank 30e having an inner-peripheralwall 32e which is brought closer to the shape of the regular trianglemore definitely. In a case where the inner roller 30 is fabricated ofbearing steel by way of example, the heat treatment may be a knownexpedient in which quenching is followed by tempering.

Lastly, as shown in FIG. 4E, the outer periphery of the inner-rollerblank 30e is subjected to a finishing work by grinding, similarly to theprior-art method. Then, the inner roller 30 is completed.

Owing to the adoption of the fabricating method thus far explained, theinner-peripheral wall 32 of the inner roller 30 is made themirror-finished surface by the burnishing work, so that the frictionfactor of the inner-peripheral wall 32 becomes small enough to expectthe smooth slip of the inner roller 30 with respect to the inner pin 7.Moreover, the surface of the inner-peripheral wall 32 is made high inhardness and high in strength by the heat treatment, so that thedurability of the inner roller 30 is enhanced.

In addition, although the "regular triangle" has been employed as the"regular polygon of n sides" in the foregoing embodiments, it sufficesas the purport of the present invention that the inner pin is permittedto equally touch all the sides of the regular polygon having the nsides. It is accordingly obvious that, as long as the number n is theinteger of at least 3, the present invention can be incarnated withoutbeing restricted to the regular triangle.

Furthermore, according to above purport of the present invention, in thepresent invention, a rhombus (namely, practically ellipse) may well beadopted.

As shown in FIG. 5, an inner-roller 30' the inner-peripheral wall 32d'of which is formed in a shape which is brought closer to a rhombus (orellipse) can be incarnated, for example, by chucking at only opposite 2(two) circumferential positions P1' and P2'.

Besides, although the structure and fabricating method according to thepresent invention have been described by taking the inner roller as theexample, it is needless to say that the present invention is alsoapplicable to the outer roller quite similarly.

As set forth above, an inner roller (or outer roller) according to thepresent invention or the inner roller (or outer roller) fabricated by amethod according to the present invention has an inner-peripheral wallthe cross-sectional shape of which is closer to a regular polygon of nsides, so that lubricating oil can be sufficiently reserved near thevertices of the regular polygon of n sides. Moreover, an inner pin (orouter pin) can be held coaxial with the inner roller (or outer roller)without giving rise to appreciable backlash. Accordingly, thereservation of the lubricating oil (the upkeep of a durability) and therelief of angular backlash can be made consistent.

Further, even when a force causing the axes of the inner pin (or outerpin) and the inner roller (or outer roller) to deviate, has been exertedby an assemblage error, an external load or the like, such a conditioncan be flexibly coped with because the touch positions of both themating members may merely shift to some extent. That is, even when suchcircumstances have occurred, favorable slip characteristics can be keptup yet, and increase in the loss of power is not incurred.

What is claimed is:
 1. An inner roller in the shape of a cylinder, foruse in an internal-meshing planetary gear construction having a firstshaft, an eccentric element which is rotated by rotation of the firstshaft, an external gear which is assembled so as to be rotatableeccentrically to the first shaft through the eccentric element, aninternal gear with which the external gear is in internal mesh, and asecond shaft which is connected to the external gear through means fortransmitting only a rotational component of the external gear on an axisthereof, said inner roller being used with an inner pin in the shape ofa column in order to constitute said means for transmitting only therotational component of the external gear on the axis thereof;characterized in that:said inner roller in said internal-meshingplanetary gear construction comprises the fact that a cross section ofan inner-peripheral wall of the cylinder is formed in a shape which isbrought closer to a regular polygon of n sides from a true circle, nbeing an integer of at least
 3. 2. An inner roller in the shape of acylinder, for use in an internal-meshing planetary gear constructionhaving a first shaft, an eccentric element which is rotated by rotationof the first shaft, an external gear which is assembled so as to berotatable eccentrically to the first shaft through the eccentricelement, an internal gear with which the external gear is in internalmesh, and a second shaft which is connected to the external gear throughmeans for transmitting only a rotational component of the external gearon an axis thereof, said inner roller being used with an inner pin inthe shape of a column in order to constitute said means for transmittingonly the rotational component of the external gear on the axis thereof;characterized in that:said inner roller in said internal-meshingplanetary gear construction comprises the fact that a cross section ofan inner-peripheral wall of the cylinder is formed in a shape which isbrought closer to a rhombus from a true circle.
 3. An outer roller inthe shape of a cylinder, for use in an internal-meshing planetary gearconstruction having a first shaft, an eccentric element which is rotatedby rotation of the first shaft, an external gear which is assembled soas to be rotatable eccentrically to the first shaft through theeccentric element, an internal gear with which the external gear is ininternal mesh, and a second shaft which is connected to the externalgear through means for transmitting only a rotational component of theexternal gear on an axis thereof, said outer roller being used in orderto constitute an internal tooth of said internal gear; characterized inthat:said outer roller in said internal-meshing planetary gearconstruction comprises the fact that a cross section of aninner-peripheral wall of the cylinder is formed in a shape which isbrought closer to a regular polygon of n sides from a true circle, nbeing an integer of at least
 3. 4. An outer roller in the shape of acylinder, for use in an internal-meshing planetary gear constructionhaving a first shaft, an eccentric element which is rotated by rotationof the first shaft, an external gear which is assembled so as to berotatable eccentrically to the first shaft through the eccentricelement, an internal gear with which the external gear is in internalmesh, and a second shaft which is connected to the external gear throughmeans for transmitting only a rotational component of the external gearon an axis thereof, said outer roller being used in order to constitutean internal tooth of said internal gear; characterized in that:saidouter roller in said internal-meshing planetary gear constructioncomprises the fact that a cross section of an inner-peripheral wall ofthe cylinder is formed in a shape which is brought closer to a rhombus atrue circle.